Process for removing free nitrogen from steel

ABSTRACT

Rimming steel sheet is annealed in proximity to an aluminium-killed steel sheet to remove free nitrogen from the former.  The two types of sheet may be alternated, or a sheet of killed steel may be clamped between two sheets of rimming steel; alternatively the two types may be formed into a common coil.

United States Patent 3,203,837 PRUCES'S FOR REMOVING FREE NITROGEN FROM STEEL Wouter J. 'G. Mees, Driehnis, Netherlands, assignor to Koninklijke Nederlandsche Hoogovens en Staalfabrieken N.V., Ijmuiden, Netherlands, a Dutch corporation No Drawing. Filed Nov. 27, 1962, Ser. No. 240,457

Claims priority, application Netherlands, Dec. 1, 1961,

14 Claims. c1. '148-16) The invention concerns a process for the reduction of free nitrogen in steel to a very low level.

It is known that effervescent, or rimmed steel has the advantage over killed steel that at a lower cost, a higher output of the former is obtainable. However, effervescent steel in general has the disadvantage of having no ageing resistance. It is also known that the ageing of effervescent steel (by which the deep-drawing characteristics of cold rolled sheets is meant) decreases in the course of time, which is a consequence of the presence of free nitrogen in this steel. In the production of killed steel, additives are added to the steel in a liquid condition, which additives have the property of forming nitrides to bind the free nitrogen. The nitrogen present in the steel is in a dissolved state and subject to a partial nitrogen pressure. It is also known that iron forms a nitride which, however, has a relatively high partial ni trogen pressure, so that at low nitrogen levels in the steel the iron nitride decomposes easily.

Because of the desirability of reducing the nitrogen content after the steel has been rolled, it is known to anneal the steel sheet in a vacuum or in an atmosphere of hydrogen or hydrogen and water vapor (moist hydrogen). In a vacuum the nitrogen migrating from the steel sheet is continuously pumped away, whereas in a nitrogen-containing atmosphere the free nitrogen pressure in the atmosphere is reduced by the formation of ammonia, which also is pumped away.

It is also known to keep effervescent steel during solidification for some time at a temperature just below the solidification temperature. This results in a sharp reduction of the nitrogen content of the steel, the nitrogen escaping in the form of a gas. With this method, however, it has not been possible to reduce the nitrogen level to such a low level that steel sheet rolled from this material'is ageing resistant. The level of the free nitrogen should be reduced to a level as low as about 0.0005% nitrogen. This percentage, however, is very diflicult to establish in view of the difficulty of analysing the nitrogen content with sufiicient accuracy at these low levels.

An object of the invention is to provide a method in which the nitrogen content of the efiervescent steel is reduced to such an extent that the steel becomes ageing resistant, the method being less expensive than known ones.

To this effect the effervescent steel sheet is brought in close contact or substantially in close contact with a substance (solid or liquid) having a low partial pressure of nitrogen. The nitrogen in the substance may be present in a dissolved state, or the substance may form nitrides.

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Another object of the invention is to provide a method wherein steel sheet containing an impurity is brought into contact with a steel sheet having a lower partial pressure of this impurity during annealing. After annealing, the properties of the steel sheet having the lower partial pressure of the impurity are not changed appreciably.

As an example of the last mentioned object an effervescent steel sheet may be annealed in contact or substantially in contact with an aluminum-killed steel sheet. In the aluminum killed steel an excess of [aluminum is present in the form of a solid solution in the steel. The partial nitrogen pressure of aluminum nitride is much lower than the partial nitrogen pressure of free nitrogen in steel. By this means the nitrogen is bound by the metallic aluminum in the aluminum-killed sheet.

Since aluminum-killed steel is among the products normally manufactured in a steel foundry, it is of very great importance that this aluminum-killed steel is exceptionally well suited to act as a nitrogen binding medium. During tests it has been proved that certain characteristics of the aluminum-killed steel change slightly owing to the nitrogen absorption, but only to so slight an extent that, for instance the very important ageing resistance of the steel is not affected. Apart from the fact that the effervescent steel sheet acquires ageing resistance, the aluminum-killed steel remains practically as marketable a product as before the annealing process. For the production of steel having ageing resistance, therefore, no more than at most one half of the steel has to be aluminumkilled.

Tests have also proved that aluminum-killed steel sheets may be used more than once as annealing agents, with effervescent steel sheet, before the ageing resistance of the former are affected noticeably.

Tests have been run with effervescent steel sheet and aluminum-killed steel sheet in close contact, the sheets having been clamped together. Tests have also been run with similar steel sheets, a gap of approximately 0.04 inch being left open between the sheets. The results of the two tests did not show significant differences between them. The importance of this feature lies in the fact that the nitrogen, or other impurity capable of migrating through the steel is in a gaseous state.

Bringing the sheets close together may be done by composing a pile of alternating effervescent and alumimum-killed steel sheets and clamping these sheets together,

In this connection reference may be made, for example,

or by coiling two sheets into one another.

Regarding the annealing temperature, the temperature preferably chosen lies close to the transition temperature of austenitic to ferritic steel.

Tests have also been run coating effervescent steel sheet in some cases with zinc and in other cases with magnesium. Results obtained are fully comparable with results obtained using aluminum-killed steel.

First test A piece of cold-rolled (so-called full-hard) eflervescent steel sheet of extra deep-draw quality, having a thickness of 1.2 mm., was cut up into strips.

The strips were combined into a pile, in which a few strips (the test strips) were alternated with strips from an aluminum-killed steel sheet of deep-draw quality, while the other strips served as blanks. The pack was put in a clamp and annealed on top of an annealing charge in an annealing furnace, annealing time 50 hours at 700 C. After this, the strips were cold-rolled once more to an elongation of 1.4%. After the strips had been halved (to a size of x 12.5 mm.), one half was tested at once, the other half after induced ageing (15% hours at 60 C., equivalent to a storage period of 3 months at 20 C.).

NOTE.--Tested only parallel to the rolling direction.

From this test appears that in the non-aged condition the yield point, elongation, and hardness of the test strips are already considerably superior to those of the blanks. After ageing, the test material exhibits a slight decrease of elongation and of the yield point elongation, and a marked decrease of hardness (though not yet to the level of the non-aged blanks). On the other-hand the yield point and the yield point/ tensile strength ratio have remained practically constant. Among the blanks all the strips exhibit a considerable deterioration (except for the elongation). In particular a marked dilference has thus arisen between test material and blanks as regards the yield point, the yield point/tensile strength ratio, and the yield point elongation.

Second test In an annealing charge of cold-rolled eifervescent extra deepdraw steel in the form of sheets (thickness 0.9 mm.) a few aluminum-killed deepdraw steel sheets were inserted in the middle of the pack. The effervescent sheets, clamped each time between two aluminum-killed sheets, served as test sheets; a few sheets from the other part of the pile served as blanks. Annealing time 17 hours at 660680 C. Test sheets and blank sheets were coldrolled once again; elongation about 1%. Again a part was tested at once, another part afted induced ageing (15 /2 hours at 60 C.).

Number Test sheets Blanks Average Number Non- Aged N on- Aged of tests aged aged per sheet Erichson value, mm 3 11.05 11. 20 11. 11 10.70 Rockwell B hardness- 3 48 44 50 51 Yield point, kg/mlnfl- 3 21.0 21. l 20. 6 25. 8 Tensile strength, kg/mmfi. 3 32.6 32. (i 34.0 34. 2 Elongation over 50 mm,

percent 3 49. 5 49. 5 47. 45. 1 Elongation over 200 111111.,

percent- 3 34. 7 34. 6 32. 6 30. 7 Uniform elongation,

percent 3 29. 29. 5 27. 8 25. 9 Yield point elongation,

percent 3 0. 8 0. 9 0. 4 4. 1

Norm-Testing took place at right angles to the rolling direction, since in this direction the characteristics are most suspectible to ageing.

From this test it appears that in the non-aged condition the hardness, the tensile strength, and the elongation of the test sheets are already superior to those of the blanks. The Erichsen value and the yield point on the other hand are slightly inferior.

After ageing, however, the Erichsen value and the hardness of the test sheets exhibit an improvement (relaxation effect). The other properties have remained practically constant, 7

On the other hand the Erichsen value, the yield point, the yield point elongation, and the elongation of the blanks have considerably deteriorated after ageing.

During treatment in the press there is a risk of Liider lines occurring when the yield point elongation exceeds 1%. When this value exceeds 2%, Liider lines are certain to occur. On this point the two tests exhibit a characteristic difference between test material and blanks in the aged condition.

What I claim is:

1. A process for reducing the nitrogen content of rimmed steel sheet by annealing the said steel sheet substantially in contact with a solid having a lower partial nitrogen pressure than the said steel, so as to promote migration from the said steel sheet to the said solid.

2. A process for reducing the nitrogen content of rimmed steel sheet by annealing the said steel sheet substantially in contact with a solid containing at least one nitride forming element, the nitride having a lower partial nitrogen pressure than the free nitrogen in the said steel.

3. A process for reducing the nitrogen content of rimmed steel sheet by annealing the said steel sheet substantially in contact with a solid containing an element of the group consisting of aluminum, chromium, titanium, boron, vanadium, silicon, zinc and magnesium.

4. A process according to claim 3, wherein said solid comprises a second steel sheet containing an amount of material selected from said group effective for reducing the nitrogen content of said rimmed steel.

5.-A process according to claim 4, wherein said second steel sheet contains aluminum.

6. A process according to claim 4, wherein said second steel sheet contains chromium.

7. A process according to claim 4, wherein said second steel sheet contains titanium.

8. A process according to claim 4, wherein said second steel sheet contains boron.

9. A process according to claim 4, wherein said second steel sheet contains vanadium.

10. A process according to claim 4, wherein said second steel sheet contains silicon.

11. A process for reducing the nitrogen content of rimmed steel sheet by annealing the said steel substan: tially in contact with aluminum-killed steel sheet.

12. A process as defined in claim 4, wherein the annealing is carried out substantially at the transition temperature of austenitic to ferritic steel, or just below this temperature.

13. A process for reducing the nitrogen content of rimmed steel sheet by annealing the said steel sheet substantially in contact with an aluminum-killed steel sheet, the aluminum-killed steel sheet being used at least twice, for treating different steel sheets, without appreciably losing its ageing resistance.

14. A process for producing ageing resistance steel sheet in which no more than half the production is aluminum-killed sheet sheet, the. remainder being rimmed steel sheet, the sheets being annealed in alternating contact with each other, so that the rimmed steel sheet becomes ageing resistant and the aluminum-killed steel sheet does not lose its ageing resistance.

References Cited by the Examiner UNITED STATES PATENTS 2,271,242 1/42 Altenburger l48--l6 2,360,868 10/44 Gensamer 14816 DAVID L. RECK, Primary Examiner. 

1. A PROCESS FOR REDUCING NITROGEN CONTENT OF RIMMED STEEL SHEET BY ANNEALING THE SAID STEEL SHEET SUBSTANTIALLY IN CONTACT WITH A SOLID HAVING A LOWER PARTIAL NITROGEN PRESSURE THAN THE SAID STEEL, SO AS TO PROMOTE MIGRATION FROM THE SAID STEEL SHEET TO THE SAID SOLID. 